Re: IGBTs are pretty fast
- From: Terry Given <my_name@xxxxxxxx>
- Date: Thu, 02 Feb 2006 10:54:56 +1300
Tim Williams wrote:
"Terry Given" <my_name@xxxxxxxx> wrote in message news:1138754474.466290@xxxxxxxxxx
- can you show a picture of the gate waveform, measured at the igbt?
Sure. http://webpages.charter.net/dawill/Images/IGBTWaveform4.jpg 5V/div vert, 2us/div horiz, waveform before gate resistors. http://webpages.charter.net/dawill/Images/IGBTWaveform1.jpg Ditto, waveform at gate. Spikes are: http://webpages.charter.net/dawill/Images/IGBTWaveform2.jpg http://webpages.charter.net/dawill/Images/IGBTWaveform3.jpg Same vertical; 200ns/div horiz. (at gate).
Note the spikes coincide with output voltage transitions, but are of the wrong polarity with respect to voltage. (In #2, gate voltage is falling, but the pulse starts negative whereas output is rising.) Thus I took a pic of the emitter waveform, about 1mm from the package. http://webpages.charter.net/dawill/Images/IGBTWaveform5.jpg (Same phase, time and voltage scales as #1 and 4.) Period is around 150ns for the pulse, i.e. 6.7MHz.
then the pulse probably isnt "real" but it shows several important things:
- your probing technique is picking up stray fields (or perhaps even oscillating by itself, but my guess is H). dont use a ground clip, take it off and use a BNC tip adaptor (the scope probe should have come with one, at least in theory), and a bnc with a short length of tightly twisted wire. that ought to make a significant difference. a simple test is to probe to your ground clip. If you see a spike, its the probe.
oh yeah, make sure the wire you use has suitable insulation voltage rating.
read Jim Williams LT AN47, and do what he says.
- there is obviously plenty of stray field to pick up - invariably its due to the loops in your power circuitry. you cant do much about your work coil (other than ensure the current is sinusoidal), but contain the rest of your fields.
Incidentially, though the pulse appears on the transistor end of the gate drive lines, the circuit side is clean. Alright, so I'll get some PCB and solder up the drive circuitses...
its well twisted, but still....with care it should work fine, but making it work gets trickier as the gatedrive moves further away. and until it works, it destroys IGBTs :)
- the output voltage waveform looks like it has a 100V spike on the rising edge. can you zoom in on that?
That's from the current waveform. '721 is the output waveform zoomed, showing about 20V overshoot.
- can u take a pic of your scope probe setup? that spike might not be there....at these low voltages, use a coax scope probe tip adaptor, a BNC socket an a short length of tightly twisted wire, soldered directly across G-E or C-E.
The current transformer is a black ferrite toroid (probably high mu) with quadfilar wound 26AWG totalling 280T (4 x 70T in series), and a 2.8 ohm load resistor. Twisted pair leads back to the scope, with a few turns around a ferrite bead for good luck. (I know, the CT is floating, and hanging on the ground lead, so it shouldn't pick up much ground loop style hash that a ferrite bead would be used for, but so what.)
I like to use coax terminated into 50R (I have some bnc thru terminators, but a T and an end terminator are about 10x cheaper) at the scope.
you can also place a faraday shield between the CT windings - suitably insulated Cu pipe, extending a short way from either side of the toroid, and one end connected to, say, earth.
The output waveform is measured with my 10x Tek probe clipped to the output terminal at the DC side of the coupling capacitor.
- how far away is your gate drive setup? put it as close as possible (creepage/clearance notwithstanding) to the IGBTs.
Not very healthy then... about 12". I do have twisted pair, which is a Transmission Line(tm), though..
- your DC bus inductance is on the order of 500nH or so.
How can that be? The 0.1 caps are an inch from any transistor, and all together should be on the order of 100-200nH.
WAG. but the loop looks to be about 1" square, so a perimeter of 4". if we use 0.01" for the "wire" thickness,
L = 0.00508*4"*[ln(4*4"/.01")-2.853] = 100nH
OK, 200nH total :)
so using the same IGBTs and bus caps, you can halve the total inductance.
if you get a couple of pieces of double-sided Cu-clad PCB, and some 1mm nomex/lexan you can make a DIY multi-layer PCB. just folding your existing assembly flat would make a large difference.
So the strips are flat with the heatsink y'mean?
yes - they make parallel-plate transmission lines, which have very low inductance - Uo*spacing/width
Ye Gods - look at the gate paralleling resistors. they form a great big loop,which (like your scope probe) *will* pick up any and all stray H field. remmeber the gate is just a cap, so a low Rg doesnt really help much here - and besides the inductance of the loop (and Tx line) also increases Zg, and the gate drive output impedance will also rise with frequency (it looks inductive).
you have to be careful with your power paths, but *PARANOID* with gate drives. I have had loops smaller than this cause fatal problems, and it wouldnt surprise me if that is the case here.
how fast is your scope? whats its rise-time spec?
SFA. The output waveform has no difference vs. vertical bandwidth (switchable 20/100/200MHz). The oscillographs were taken at 100MHz. All the same, it is spec'd at 1.8ns or so.
thats fast enough.
an HP 17xx?
Tim
Cheers Terry .
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